denis
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Acid/Base Chem Equations
I am utterly lost with the chemical equations for strong acids/bases.
For a strong base (HCl) the (dissociation) equation would be:
HCl(aq) + H2O(L) -> Cl(aq) + H3O(aq)
but for a stong base (Sr(OH)2) it would be:
Sr(OH)2 --H2O--> Sr + 2OH
Why is the H2O not included in the equation for strong bases (instead on the arrow) but on a strong acid, the H2O is included?
Sorry if this is a lame question, I just need help badly 
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Nicodem
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Acid/base chemistry is about transfers of protons from one chemical species to the other (by Bronstein definition) or electron pair donation from one
chemical species to the other (a wider definition by Lewis). I suggest you to use the Lewis concept since it is more general and actually makes more
sense:
http://en.wikipedia.org/wiki/Acid-base_reaction
http://en.wikipedia.org/wiki/Lewis_acid
http://en.wikipedia.org/wiki/Br%C3%B8nsted-Lowry_acid-base_t...
etc.
Your equation about HCl reaction with H2O is obviously wrong since you wrote it as a homolytic dissociation. Always remember that any acid/base
chemistry involves heterolytic bond dissociations and/or formation. It is always a pair of electrons that migrates, never a single electron. In this
specific case the HCl dissociates heterolyticaly in water:
HCl + H<sub>2</sub>O <-> Cl<sup>-</sup> + H<sub>3</sub>O<sup>+</sup>
Acid/base reactions are equilibrium reactions with the difference in pKa's of involved acids determining the equilibrium constant (see http://www.lauher.com/che142/acidbase.pdf).
For example, the pKa of HCl in water is about -7 while that of H<sub>3</sub>O<sup>+</sup> in water is -1.74. Thus the
equilbrium constant of the reaction is about 10<sup>5,26</sup>=5.5*10<sup>6</sup> (the "p" in pKa stands for the negative
logarithm function (-log), therefore K=10<sup>-pKa</sup> . This is a
very large equilibrium constant and actually means that the reaction is leaned far to the right side toward the chloride and hydronium ions.
Your second equation is also wrong for similar reasons.
In these equations it is always implied the ionic species are solvated hence they only are true for diluted solutions where solvation is
complete. For example, a 10M HCl is anything but completely dissociated since there is a deficit of H2O molecules to efficiently solvate all the
chloride and hydronium ions, the polarity/dielectric constant of such mixture is also different from that of pure water, and therefore the pKa of HCl
is not -7 any more in concentrated solutions. (see http://en.wikipedia.org/wiki/Solvation)
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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denis
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Thats sort of beyond what im learning (High School/Grade 12 - Canada), the pKa stuff I mean, unless we're still going to be learning about it later, I
dont know.
So HCl forms an equilibrium in water?
If so..their just teaching us wrong/useless stuff, to simply make it less confusing (but more confusing later on - college/university).. Just like how
last year they taught us Standard Temp. Pressure and now we never use it (only SATP), and converting grams to # of particles (w/avagadros number), now
we never use that either. Pointless education at its best... they never teach stuff we'll use later?
Basically right now we being taught that HCl and water will dissociate 100%, so that a solution of HCl is actually made up from H3O+ and Cl- ions..is
that even partially correct, or later on will I find it is wrong? Dont tell me why its wrong/whats right, just yes or no, because ill probably mess up
my next test from all the confusion 
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chemrox
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I think I see your confusion. Water is involved in both because these are aqueous equilibria. However the basic species, OH- is formally an entity
in this paradigm where as there is no such thing as H+. The acidic proton is always shown associated with a water molecule as H3O+. When I learned
this system we used H+. HCl = H+ + Cl- for the dissociation of acids and NaOH = Na+ + OH- for the dissociation of bases. Often the expression (aq)
was added to show that these are taking place in water. Your second expression doesn't balance. Go back and look again. Sr(OH)2 (aq) = Sr++ (aq) +
2(OH-) (aq)
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Nicodem
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| Quote: | Originally posted by denis
Thats sort of beyond what im learning (High School/Grade 12 - Canada), the pKa stuff I mean, unless we're still going to be learning about it later, I
dont know.
So HCl forms an equilibrium in water?
If so..their just teaching us wrong/useless stuff, to simply make it less confusing (but more confusing later on - college/university).. Just like how
last year they taught us Standard Temp. Pressure and now we never use it (only SATP), and converting grams to # of particles (w/avagadros number), now
we never use that either. Pointless education at its best... they never teach stuff we'll use later? |
I don't know the educational system in Canada, but yes, even in my country we only briefly explain the pKa concept, reaction equilibriums and
solvation to the pupils in what would be equivalent to high school. It is normal that you don't learn the concepts that are useful in research since
you are not expected to be involved in research while in high school. You are thought the very basic stuff in a very, very simplified manner with
lot's of approximations. For example, that they tell you the dissociation of HCl in water is 100% is not after all completely wrong. In an idealized
model of high dilution you can approximate it to 100% since the undissociated form of HCl will only be present in the order of parts per million. This
way of teaching is only wrong when you take it too seriously and don't consider it as a simplified theory. A theory is always a simplification of
reality and needs not to be true anyway - there is always place for upgrading or even completely modifying it. For example, the Newton mechanics
theory works perfectly and 'exactly' in all the cases, except for the smallest processes of low energy and low mass (of the order of eV and electron
mass!) or extreme velocities (like approaching the speed of light), but people deal with flying baseballs more commonly than flying electrons or
speeds in the range of hundreds of thousands kilometres per second in their normal life learning.
| Quote: | | Basically right now we being taught that HCl and water will dissociate 100%, so that a solution of HCl is actually made up from H3O+ and Cl- ions..is
that even partially correct, or later on will I find it is wrong? Dont tell me why its wrong/whats right, just yes or no, because ill probably mess up
my next test from all the confusion |
Now they teach you so, but if you ever go to study chemistry you will later on find out that there are no such ions in water solutions. All ions
interact with H2O molecules in a complicated manner. For example, you don't have "naked" Cl<sup>-</sup> anions in water, they are always
solvated with H2O molecules trough hydrogen bonding and the net charge is neutralized trough solvent dipole orientation around the ion. Such solvated
ions are impossible to characterize since the interaction is dynamic (no stable single species of hydrated ions form), therefore it is much easier to
ignore such a state of things and teach at a more basic and what appears a terribly simplified level. However, trust me, at your age you don't want to
learn these things at the top level of the theory.
But if you would ever decide to study chemistry you will have to learn these concepts in more depth or else you will never understand why is
Cl<sup>-</sup> such a poor base in water, why does it have a poorer nucleophilicity in water than Br<sup>-</sup> and
I<sup>-</sup> and why this is the opposite in aprotic solvents where Cl<sup>-</sup> is a stronger nucleophile than the other
two...
Like Chemrox said, you can be happy they don't teach you that HCl dissociates to Cl<sup>-</sup> and H<sup>+</sup> like they
used to do only a few decades ago.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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woelen
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The problem with teaching such simplified models is not that they are simplified. Actually, that is good, otherwise young people never would grasp the
subject. My problem is that these simplified models are presented as the whole story.
As a young boy I was teached all kinds of precipitation reactions, such as CuCl2 + 2NaOH --> Cu(OH)2 + 2NaCl. Now I see that this is a very strong
simplification (in reality all kinds of mixed chloride/hydroxide/hydrated species occur and hence the color of the precipitate is not nice blue, but
some greenish stuff is formed). These models were presented as the complete thing.
So, what I really would like to see is that young people are told also that the models they are learning are not the whole story, but simplifications,
introduced to them in order to make picking up the subject feasible. A more complete picture will emerge at a later age and a complete picture will
never be given, because it does not exist (this is the nature of natural sciences, any model, no matter how detailed, is an approximation of reality).
[Edited on 20-11-08 by woelen]
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chemrox
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I hope this is the case.. it's how I learned. Ideal Gas law comes to mind. The real lesson in learning to do chemistry calculations is a method of
analytical thinking. I learned how to think in my first college chemistry courses. Along with equilibria, kinetics, dimensional analysis and all the
rest, the real lesson ws how to think about a problem and arrive at possible solutions based on data and theory.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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